Jejunoileal bypass (JIB) patients provide a unique opportunity to study the relationship between small bowel loss and gastric secretory function. Preoperatively, and approximately one year postoperatively, measurements of gastric secretion were taken in 37 patients who underwent JIB for massive obesity. Basal acid output increased by 0.52 ± 0.35 mEq/hr (P>.2), and peak stimulated acid output increased by 1.99 ±0.96 mEq/30 min (P <.05). A separate group of 26 postoperative and 17 preoperative (control) JIB patients had fasting serum gastrin levels measured by radioimmunoassay. Postoperative patients had levels of 37 ± 5 pg/ml, and control patients had levels of 36 ± 4 pg/ml (P>.5). We conclude that following JIB for obesity, there is no significant change in basal acid secretion or in serum gastrin. There is a small, but statistically significant, increase in peak stimulated acid output. We currently find no clinical correlation with this change.